Process for producing aluminum oxide films at low temperatures

ABSTRACT

A process for producing aluminum oxide thin films on a substrate by the ALD method comprises the steps of bonding a vaporizable aluminum compound to a growth substrate, and converting the bonded organoaluminum compound to aluminum oxide. The bonded aluminum compound is converted to aluminum oxide by contacting it with a reactive vapor source of oxygen other than water, and the substrate is kept at a temperature of less than 190° C. during the growth process. By means of the invention it is possible to produce films of good quality at low temperatures. The dielectric thin films having a dense structure can be used for passivating surfaces that do not endure high temperatures. Such surfaces include, for example, polymer films such as organic electroluminescent displays. Further, when a water-free oxygen source is used, surfaces that are sensitive to water can be passivated.

BACKGROUND OF THE INVENTION

[0001] The present application claims priority under 35 U.S.C. §120 as acontinuation of U.S. patent application Ser. No. 10/003,749, filed Oct.23, 2001 and under §119(a) to Finnish Patent Application No. 20002323,filed Oct. 23, 2000.

[0002] 1. Field of the Invention

[0003] The present invention concerns a process for producing aluminumoxide films by an ALD type process. According to the preferred processthe aluminum oxide films are produced at low temperatures by bonding ametal compound on a substrate and converting said metal compound into ametal oxide.

[0004] 2. Description of Related Art

[0005] Dielectric thin films with a high dielectric constant(permittivity) have a number of applications in the field ofmicroelectronics. For example, they may replace the SiO₂ and Si₃N₄presently used in DRAM-memories in order to maintain the necessarycapacitance as the size of capacitors is reduced.

[0006] Al₂O₃ films suitable for passivating surfaces have previouslybeen prepared by physical processes, such as sputtering. The problemwith the films produced by sputtering has been the unevenness of theresulting film, and the pinholes that are formed in the film. Thesepinholes may form a diffusion path for water through the film.

[0007] U.S. Pat. No. 6,124,158 discloses a method of reducing carboncontamination of Al₂O₃ thin films deposited by an ALD method. The ALDprocess uses organic aluminum precursors and water. In at least everythird cycle, ozone is introduced into the reaction chamber to reducecarbon contaminants. The process has its limits since aluminum oxidefilms deposited below 190° C. were not dense or reproducible.

[0008] ALD methods have also been used for producing Al₂O₃ films byusing aluminum alkoxides, trimethyl aluminum (TMA) or AlCl₃ as thealuminum source material and water, alcohols, H₂O₂ or N₂O as the oxygensource material. Al₂O₃ films from TMA and water have been deposited attemperatures in the range of 150° C. to 400° C. Typically thetemperature has been between 150° C. and 300° C. The resulting films haduniform thickness and did not contain any pinholes. However, the densityof the film has been questionable at the lower end of the depositiontemperature range.

[0009] In applications involving organic polymers or low molecularweight organic molecules, such as organic EL displays, the depositiontemperature is preferably less than 150° C., while in applications wherethe substrate is sensitive to water it is not feasible to use water asthe oxygen source material. Thus there is a need for a process ofproducing aluminum oxide films by ALD at low temperatures using anoxygen source other than water.

SUMMARY OF THE INVENTION

[0010] The present invention is based on the surprising finding thathigh-quality aluminum oxide thin film can be grown by an ALD typeprocess at substrate temperatures down to 100° C. Another surprisingfinding is that ozone can be used in the deposition process withoutdestroying the properties of substrates that contain an organic layer. Adense, pinhole-free thin film layer can be produced very quickly by ALDon the substrate surface, which protects the sensitive materialsunderneath the surface against the surrounding gas atmosphere.

[0011] In accordance with one aspect of the present invention a processis provided for depositing a thin film of aluminum oxide on a substrateby an atomic layer deposition process that comprises a plurality ofcycles. Each cycle comprises supplying a first reactant that comprises agaseous aluminum compound and supplying a gaseous second reactant thatcomprises a source of oxygen other than water. The second reactantconverts the adsorbed portion of the first reactant on the substrate toaluminum oxide. Preferably the substrate is maintained at a temperatureof less than 190° C. during the ALD process.

[0012] A number of considerable advantages are obtained by means of thepreferred embodiments. For example, with the aid of the presentinvention, it is possible to produce Al₂O₃ films of good quality at lowtemperatures.

[0013] Dielectric thin films with a dense structure can be used forpassivating surfaces that do not tolerate high temperatures. Suchsurfaces include, for example, polymer films. If a water-free oxygensource is used, surfaces that are sensitive to water can also bepassivated.

[0014] In addition, dielectric films, including aluminum oxide thinfilms, with a dense structure can be used as buffer layers betweenfunctional films that include at least one organic film. The dielectricfilm then prevents either reaction or diffusion between the functionalfilms.

BRIEF DESCRIPTION OF THE FIGURES

[0015]FIG. 1 is a schematic depiction of a cross-section of an organicEL display which is sealed against the surrounding gas atmosphere with apassivating layer.

[0016]FIG. 2 depicts a structure of a SAW (Surface Acoustic Wave)filter.

[0017]FIG. 3 depicts the structure of preferred peracids, such asperacetic acid CH₃COOOH, which contain OOH and O groups bound to thesame carbon atom

[0018]FIG. 4 depicts the structure of preferred organic ozonides, whichcontain both O and O—O groups between two carbon atoms.

[0019]FIG. 5 depicts the elimination of surplus OH groups and formationof Al—O—Al bridges that may lead to the densification of Al₂O₃ surfaces.

DETAILED DESCRIPTION OF THE INVENTION Definitions

[0020] For the purpose of the present invention, an “ALD type process”designates a process in which deposition of a thin film from gaseoussource chemicals onto a substrate surface is based on sequentialself-saturating surface reactions. The principles of the ALD process aredisclosed, e.g., in U.S. Pat. No. 6,015,590.

[0021] “Reaction space” is used to designate a reactor or reactionchamber in which the conditions can be adjusted so that the depositionof a thin film on a substrate by ALD is possible.

[0022] “Thin film” is used to designate a film that is grown fromelements or compounds that are transported as separate ions, atoms ormolecules via vacuum, gaseous phase or liquid phase from the source tothe substrate. The thickness of the film depends on the application andit varies in a wide range, e.g., from one molecular layer (about 5 nm)up to 1,000 nm, or even more.

[0023] By “dense” structure is meant a thin film which has a smallerleakage current through it, or which has a lower permeability to ions orgases when comparing two thin films consisting essentially of the samebasic material, such as a thin film of Al₂O₃ deposited by the process ofthe present invention and a thin film of Al₂O₃ deposited using water.

The Deposition Process

[0024] An aluminum oxide thin film functioning as a dielectric or apassivating layer is grown on a desired substrate by ALD. The substrateis heated to the processing temperature, which is preferably selectedfrom a range of about 100° C. to 190° C. More preferably the processingtemperature is less than 150° C., even more preferably less than 140° C.and even more preferably less than 130° C.

[0025] In particular, the following organoaluminum compounds containingat least one alkyl group bound to aluminum may be used in the presentinvention:

[0026] Monoalkyl aluminum compounds L¹AlX₂ wherein X is selected fromthe group consisting of H, F, Cl Br, I, RCHO, wherein RCHO is an alkoxygroup and L¹ is a linear or branched hydrocarbon that is saturated orunsaturated.

[0027] Dialkyl aluminum compounds L¹L²AlX wherein X is selected from thegroup consisting of H, F, Cl, Br, I, RCHO, wherein RCHO is an alkoxyligand and L¹ and L² are linear or branched hydrocarbons with single,double and/or triple bonds.

[0028] Trialkyl aluminum compounds L¹L²L³Al wherein L¹, L² and L³ arelinear or branched hydrocarbons with single, double and/or triple bonds.

[0029] The organoaluminum compound is preferably introduced into thereaction chamber in gaseous phase and contacted with the substratesurface.

[0030] Most preferably trimethyl aluminum (CH₃)₃Al, also known as TMA,is used as the aluminum source chemical.

[0031] Strongly oxidizing source chemicals containing oxygen arepreferably used in the process. One or several chemicals selected fromthe following group are preferably used as an oxygen source: ozone,organic ozonides, oxygen atoms containing unpaired electrons, organicperoxides and organic peracids.

[0032] Preferred peracids, such as peracetic acid CH₃COOOH, contain OOHand O groups bound to the same carbon atom as depicted in FIG. 3.

[0033] Preferred organic ozonides contain both O and O—O groups betweentwo carbon atoms, as depicted in FIG. 4.

[0034] Dimethyl peroxide and benzoyl peroxide are examples of suitableorganic peroxides. In addition, other preferred peroxides includeperoxides selected from the following group: R¹—O—O—R², wherein R¹ andR² are linear, branched or cyclic organic ligands such as CH₃, (CH₃)₃C,C₆H₅ or benzoyl; and

[0035] R¹—O—O—H, wherein R¹ is a linear, branched or cyclic organicligand such as CH₃, (CH₃)₃C or C₆H₅.

[0036] Most preferably ozone is used as an oxygen source. Water ispreferably not used as a source chemical in the deposition process. Theresulting growth rate of aluminum oxide may be as good as around 0.8Å/cycle. It is assumed that the surface reactions between ozone andtrimethyl aluminum or fractions of trimethyl aluminum bound to surface,such as dimethyl aluminum and monomethyl aluminum, provide enough OHgroups on the aluminum oxide surface for the self-saturatingchemisorption reaction of the following trimethyl aluminum pulse withthe substrate surface.

[0037] In addition to serving as an oxygen source for the process, ozonealso contains a lot of chemical energy that is released when themolecule is broken.

O₃(g)→3/2O₂(g)ΔH_(f) ⁰=−142.7 kJ/mol and ΔG _(f) ⁰=−163.2 kJ/mol

[0038] (N. N. Greenwood and A. Earnshaw, “Chemistry of the Elements”,Pergamon Press Ltd., Oxford, England 1986.)

[0039] Break up of ozone molecules can provide additional energy to thetop molecular layers of surfaces and thus promote some surfacereactions. The densification of Al₂O₃ surface may proceed viaelimination of surplus OH groups and formation of Al—O—Al bridges asdepicted in FIG. 5.

[0040] Also when the O—O bond of an organic peroxide breaks, theresulting RO fragments are highly reactive.

[0041] Before introducing ozone into the reaction chamber, the ozone isoptionally diluted. Oxygen gas, inert gases such as nitrogen, or noblegases such as argon may be used for this purpose.

[0042] Examples of applications for which aluminum oxide films preparedaccording to the present process are particularly suitable are organiclight-emitting diodes (OLED), organic electroluminescent displays (OEL),organic solar cells (OSC) and surface acoustic wave (SAW) filters. Theseapplications generally require a low deposition temperature and/or aresensitive to moisture and/or oxygen.

[0043] According to a first preferred embodiment of the presentinvention, an organic EL display is provided with a passivating layeraccording to the preferred deposition process. A typical organic ELdisplay is produced by arranging an anode 12 on a substrate 11 (cf.FIG. 1) that is typically made of glass or a similar material. A holetransport layer 13 is deposited on the anode 12 and an emission layer 14is deposited on the hole transport layer 13. Further, a layer 15 capableof transporting electrons is deposited on the emission layer 14. Allthese layers 13-15 preferably comprise organic material. The organicmaterial may be polymeric or low molecular weight molecules. A cathode16 is then formed on the layer 15 capable of transporting electrons. Thecathode 16 is preferably made of a metal such as aluminum, magnesium orcalcium-coated aluminum. These metals easily acquire an oxide layer onthe surface that may be detrimental to the interface between the metaland the organic layer. A passivating layer 17 is therefore deposited onthe surfaces of the resulting structure by the present process. It is tobe noted that “surfaces” means all the possible surfaces. Thus verticalsurfaces are also preferably passivated.

[0044] According to a second preferred embodiment of the presentinvention, a SAW filter is provided with a protective layer according tothe preferred deposition processes. A typical SAW filter is presented inFIG. 2. It comprises a first acoustic absorber 21 and a second acousticabsorber 22 placed on a piezoelectric substrate that is typicallyquartz, lithium niobate or lithium tantalate. The incoming signal isconducted to an input transducer 23 and the outgoing signal is collectedfrom an output transducer 24. The input transducer 23 convertselectrical signals to small acoustic waves that are reconverted toelectrical signals by the output transducer 24. Usually the structure ishermetically encapsulated. The present invention replaces the hermeticencapsulation with a thin protective layer that is deposited on thesurface of the SAW structure by the described process. Thus, a lessexpensive encapsulation process can be applied to the protectedstructure to obtain a finished SAW product.

[0045] In each of the above embodiments, the Al₂O₃ passivation layer ispreferably between about 5 nm and 1,000 nm in thickness, more preferablybetween about 25 nm and 75 nm.

[0046] Examples of ALD reactors in which the low-temperature process canbe performed include single wafer reactors, reactors with multiplewafers or other substrates in a planar or vertical substrate holder andbatch process reactors. For example, organic solar cells are preferablycoated with a protective layer by the process in a batch process reactorto keep the manufacturing costs per substrate low.

[0047] The invention is further illustrated with the aid of thefollowing non-limiting working examples.

EXAMPLE 1 Depositing Al₂O₃ Thin Films Using Either Water or Ozone as anOxygen Source

[0048] Case A: Deposition of Al₂O₃ Films with Water as an Oxygen Source

[0049] Al₂O₃ thin films were deposited in a flow-type ALD reactor, modelF-120, manufactured by ASM Microchemistry Oy, Finland. Trimethylaluminum (CH₃)₃Al, also known as TMA, was used as an aluminum sourcechemical. Purified water was used as an oxygen source chemical. Thesource chemicals were introduced from external sources to the reactor.

[0050] A substrate was provided to the reaction space and the reactorwas evacuated to vacuum with a mechanical vacuum pump. Next, thepressure of the reaction space was adjusted to a range of approximately5-10 mbar with flowing nitrogen gas. Then the reaction space was heatedto the deposition temperature.

[0051] Thin films were deposited at either 100° C. or 300° C. The sourcechemicals were pulsed into the reaction space according to theprinciples of ALD, e.g. the pulses were separated from each other withinert gas to prevent mixing of the source chemicals in the gas phase ofthe reaction space. Only surface reactions were allowed to occur.

[0052] The pulsing cycle was as follows: TMA pulse 0.5 s N₂ purge 1.0 sH₂O pulse 0.4 s N₂ purge 1.5 s

[0053] The growth rate of Al₂O₃ from TMA and H₂O was 0.8 Å/cycle at 300°C. and 0.5 Å/cycle at 100° C. The refractive index was 1.64 for the filmgrown at 300° C. and 1.59 for the film grown at 100° C. The films grownat 100° C. started to leak immediately in the electrical measurementsand it was impossible to measure exact values for capacitance orbreakdown voltage. It appeared that the films were not very dense. Asummary of the properties is presented below in Table 1.

[0054] Case B: Deposition of Al₂O₃ Film with Ozone as an Oxygen Source.

[0055] Al₂O₃ thin films were deposited in a flow-type ALD reactor, modelF-120, manufactured by ASM Microchemistry Oy, Finland. Trimethylaluminum (CH₃)₃Al, also known as TMA, was used as an aluminum sourcechemical. Ozone prepared on the premises was used as an oxygen sourcechemical. The source chemicals were introduced from external sources tothe reactor.

[0056] A substrate was provided to the reaction space and the reactorwas evacuated to vacuum with a mechanical vacuum pump. Next, thepressure of the reaction space was adjusted to a range of approximately5-10 mbar with flowing nitrogen gas. Then the reaction space was heatedto the deposition temperature.

[0057] The thin films were deposited at either 100° C. or 300° C. Thesource chemicals were pulsed into the reaction space according to theprinciples of ALD as in case A.

[0058] The pulsing cycle was as follows: TMA pulse 0.5 s N₂ purge 1.0 sO₃ pulse 4.0 s N₂ purge 1.5 s

[0059] A summary of the properties of the resulting thin films ispresented below in Table 1. TABLE 1 Case B Case B Case A Case ADeposition temperature 100° C. 300° C. 100° C. 300° C. Growth rate (Åcycle) 0.8 0.8 0.5  0.8  Refractive index  1.58  1.66 1.59 1.64Dielectric constant 6.0 8.3 * Breakdown voltage (MV/cm) 4.5 6.0 *

[0060] A TOF-ERDA analysis of the film grown from TMA and ozone at 100°C. revealed that the film contained 6.0% carbon and 15.8% hydrogen.

[0061] A comparison between Case A and Case B shows that replacing waterwith ozone was beneficial for the deposition process at low temperature.

EXAMPLE 2 Depositing Al₂O₃ Thin Films on an Organic Layer Using Ozone asan Oxygen Source

[0062] A substrate with an organic thin film was provided into thereaction space of an F-450 ALD reactor manufactured by ASMMicrochemistry Oy, Finland. The pressure of the reaction space wasadjusted to about 5-10 mbar with a mechanical vacuum pump and flowingnitrogen gas that had a claimed purity of 99.9999%. Then the temperatureof the reaction space was adjusted to about 110° C. TMA evaporated froman external source and ozone prepared on the premises were alternatelyintroduced into the reaction space and contacted with the surface. Thepulsing times were 1 s for TMA and 4 s for O₃. The source chemicalpulses were separated from each other with nitrogen gas. The purgingtime lasted for 1.0-1.5 s after each source chemical pulse. The pulsingcycle consisting of these two source chemical pulses and two purgingperiods was repeated until a 50 nm aluminum oxide thin film was producedon the substrate. Typically, around 600 pulsing cycles were needed forthe deposition. As a result, the deposition process did not adverselyaffect the organic layer. Further, the passivated structure could bestored in ordinary room atmosphere without destroying the functionalityof the organic layer.

[0063] Although the foregoing invention has been described in terms ofcertain preferred embodiments, other embodiments will become apparent tothose of ordinary skill in the art in view of the disclosure herein.Additionally, while one embodiment is disclosed in the context of an ELdisplay, and another embodiment is disclosed in the context of a SAWfilter, the skilled artisan will readily find application for theprinciples disclosed herein in a number of different contexts.

[0064] Accordingly, the present invention is not intended to be limitedby the recitation of preferred embodiments, but is intended to bedefined solely by reference to the claims.

We claim:
 1. A method for depositing a dielectric film on a substrate ina reaction chamber by an atomic layer deposition (ALD) processcomprising alternately providing a gaseous metal compound and an oxygensource other than water to the reaction chamber, wherein the substrateis maintained at a temperature of less than 190° C. during the ALDprocess.
 2. The method of claim 1, wherein the dielectric film comprisesa metal oxide.
 3. The method of claim 2, wherein the dielectric filmcomprises aluminum oxide.
 4. The method of claim 1, wherein the oxygensource comprises one or more compounds selected from the groupconsisting of ozone, organic ozonides, oxygen atoms with unpairedelectrons, organic peroxides and organic peracids.
 5. The method ofclaim 1, wherein the oxygen source comprises one or more organicperoxides with the formula R¹—O—O—R² (IV), wherein R¹ is a linear,branched or cyclic organic ligand, and wherein R² is hydrogen or alinear, branched or cyclic organic ligand.
 6. The method of claim 1,wherein the substrate is maintained at a temperature of less than 150°C. during the ALD process.
 7. The method of claim 6, wherein thesubstrate is maintained at a temperature of less than 140° C. during theALD process.
 8. The method of claim 7, wherein the substrate ismaintained at a temperature of less than 130° C. during the ALD process.9. The method of claim 1, wherein the dielectric film is deposited to athickness of between about 5 nm and about 1000 nm on the substrate. 10.The method of claim 9, wherein the dielectric film is deposited to athickness of about 25 nm to about 75 nm.
 11. The method of claim 1,wherein the substrate comprises an organic light emitting layer.
 12. Themethod of claim 1, wherein the substrate comprises a surface acousticwave filter.
 13. The method of claim 1, wherein the substrate comprisesan organic solar cell layer.
 14. The method of claim 1, wherein thesubstrate is sensitive to moisture.
 15. The method of claim 14, whereinthe dielectric film protects the substrate from moisture.
 16. The methodof claim 1, wherein the dielectric film is deposited on an aluminumoxide layer that was previously deposited by chemical vapor depositionor physical vapor deposition.
 17. The method of claim 1, wherein thesubstrate is sensitive to the atmosphere.
 18. The method of claim 17,wherein the dielectric film protects the substrate from the atmosphere.19. A method of passivating a substrate surface that is sensitive tohigh temperatures comprising depositing a metal oxide dielectric film onthe surface by an atomic layer deposition (ALD) process comprisingalternately contacting the substrate surface with a metal source and anoxygen source other than water, wherein the substrate is maintained at atemperature of less than 150° C. during the ALD process.
 20. The methodof claim 19, wherein the surface comprises a polymer film.
 21. Themethod of claim 19, wherein the surface comprises an organic film. 22.The method of claim 19, wherein the oxygen source is ozone.
 23. Themethod of claim 19, wherein the metal oxide comprises aluminum oxide.24. The method of claim 19, wherein the substrate is maintained at atemperature of less than 130° C. during the ALD process.
 25. A method ofpassivating a substrate surface that is sensitive to moisture comprisingdepositing a metal oxide dielectric film on the surface by an atomiclayer deposition (ALD) process comprising alternately contacting thesubstrate surface with a metal source and an oxygen source other thanwater, wherein the substrate is maintained at a temperature of less than190° C. during the ALD process.